The low rank coal, such as subbituminous coal or brown coal, which has a moisture content higher than about 20 mass % is limited to use within a coal-producing region. This is because, for example, the low rank coal has a low calorie resulting from its high moisture content and generates a small amount of heat by combustion. On the other hand, when dried, the low rank coal becomes more spontaneously combustible and more hygroscopic, resulting in transportation costs being relatively expensive, etc.
However, the low rank coal has advantages that are not found, e.g., in the bituminous coal that is considered to be a high rank coal. For example, brown coal found in Australia and Indonesia is low in sulfur content and produces less ash. Thus, using the brown coal as a fuel would make it possible to prevent air pollution due to sulfur dioxide gas, etc., as well as to reduce hazardous ash waste.
In this context, such techniques have been suggested which are adopted to carbonize the low rank coal by dehydration reform or thermal reform, thereby compensating for the drawbacks thereof. For example, disclosed in Patent Literature 1 and Patent Literature 2 is a technique by which oil and low rank coal are mixed into raw material slurry; the resulting slurry is heated and dehydrated in the oil, and then further heated to decompose or detach the carboxyl radical or the hydroxyl radical, etc., in the raw material coal by a decarboxylation reaction or a dehydration reaction, thereby reforming the raw material coal. Also disclosed is a technique by which heavy oil, etc., is penetrated into pores of the low rank coal so as to prevent spontaneous combustion.
On the other hand, disclosed in Patent Literature 3 is a coal gasification complex power generation facility which includes the following: a gasification unit for gasifying low rank coal having a relatively high moisture content; a gas power generation unit for generating electric power using a gas supplied from the gasification unit; a steam power generation unit for generating electric power by the heat of an exhaust gas emitted from the gas power generation unit; and a coal drying unit for drying coal by the exhaust heat emitted from the steam power generation unit and supplying the dried coal to the gasification unit. Disclosed in Patent Literature 4 is a method for manufacturing reformed coal and hydrocarbon oil by pyrolyzing brown coal into reformed coal and tar under an inert gas atmosphere or steam atmosphere and catalytically cracking the tar in a steam atmosphere and in the presence of an iron-based catalyst so as to obtain hydrocarbon oil.